1. Field of the Invention
This invention relates to a cleaning device for a steam generator and, more specifically, to a miniature sludge lance structured to pass between adjacent tubes in the steam generator.
2. Description of the Prior Art
A pressurized water nuclear reactor utilizes a steam generator to maintain separation of the water that passes over the nuclear fuel (the “primary water”) and the water that passes through the electricity generating turbines (the “secondary water”). The steam generator has an outer shell defining an enclosed space, at least one primary fluid inlet port, at least one primary fluid outlet port, at least one second fluid inlet port, at least one second steam outlet port, and a plurality of substantially uniformly sized tubes extending between, and in fluid communication with, the at least one primary fluid inlet port and at least one primary fluid outlet port. That is, the primary water passes through a manifold that divides the primary water into multiple streams that pass through the plurality of tubes. This manifold may be located inside or outside of the steam generator shell, but is preferably disposed inside the steam generator shell. The secondary water may also pass through a manifold, or simply multiple inlets/outlets, but is typically passed through a single inlet and a single outlet. A typical steam generator is cylindrical, about sixty feet tall and about twelve feet in diameter.
The tubes are disposed in a substantially regular pattern extending substantially vertically and having substantially uniform, narrow gaps between adjacent tubes. Further, the tubes typically have an overall shape of an inverted “U” and are coupled to a flat plate having a plurality of opening therethrough. This flat plate, or tube sheet, along with another plate that separates the at least one primary fluid inlet port and at least one primary fluid outlet port, substantially forms the manifold noted above. Thus, within the stream generator shell, the tubes have an ascending side (hot) and a descending side (cool). Between these two sides there is a gap identified as the “tube lane”. The steam generator shell has openings at various elevations and on either side of the tube lane. Typically, the openings are disposed in opposing pairs. A six inch diameter penetration for opening at the tube lane axis is typical. Since the tube lane is formed by the dome of U-shaped tubes, access to the center of the steam generator is generous along the tube lane.
In operation, the primary water is communicated through the tubes and the secondary water passes over the tubes. As this occurs, the secondary water is heated and the primary water is cooled. During operation of the pressurized water reactor steam generator, sediment is introduced on the secondary side as the secondary water changes to steam. This particulate sediment, or sludge, is deposited on most exposed surfaces including on the outer surface of the tubes and, primarily, on the top of the tube sheet. Periodic cleaning of the sediment is desirable to maintain good heat transfer and water flow in the steam generator. A typical cleaning is performed by sweeping high pressure and high volume water jets introduced along the tube lane axis of the steam generator where there is ample clearance. That is, a “lance” structured to spray high pressure water is moved through the tube lane and is structured to spray water generally laterally (i.e. generally perpendicular to the axis of the tube lane) and downwardly in between the tubes. This spray lifts most of the sludge off the tube sheet and removes sludge from the exposed sides of the tubes. The cleaning can be preceded by chemical treatment. This cleaning pattern, however, may leave sludge between the close pattern of tubes and is less effective at locations spaced from the tube lane.
It is further noted that, in order to regulate secondary side water flow patterns in the steam generator, devices called tube lane blocks have been installed in some steam generators. The tube lane blocks can prohibit access for cleaning equipment through the six inch penetration. Support plate structures (stay rods) located within the tube bundles of steam generators are other obstructions that can prevent effective cleaning. Due to various internal physical restrictions in the tube lane (the area generated along the centerline of the tube sheet by the minimum bend radius of the Row 1 tubes), the tube sheet legs (either hot or cold depending on the location of the inlet nozzle) cannot be adequately cleaned by conventional lancing equipment mounted to the hand holes. Access to the tube bundle is further restricted by an arrangement of Tube Lane Blocking Devices (TLBD's) and a Blowdown Pipe positioned directly along the centerline of the hand hole in the tube lane.
In addition to tube lane access, some steam generators have smaller inspection penetrations, openings about two inches in diameter, located at various orientations and elevations about the steam generator. After entrance through an inspection penetration, access is limited by the gap between adjacent tubes. These openings are not typically used for cleaning because the problem is to accurately position and sweep high pressure cleaning jets and deliver high water volume within the confines of adjacent tube spacing and the inspection penetration. These penetrations can also be disposed several degrees from the center of the tube lane. Sludge lancing is typically not performed through these penetrations due to their physical size and location. Therefore, the tube lane in these steam generators is basically inaccessible and prone to accumulating sludge and debris under the blowdown pipe and between the TLBD's. In addition, certain utilities have forbidden hand-lancing with static jets that impinge directly on the tube sheet and adjacent steam generator tubing—this limits certain types of manual lancing that could be employed through the inspection penetrations to clean this region. Sludge lancing technicians are subjected to higher doses or radiation with equipment that does not provide an automated mechanical means of oscillation or rotation of the high velocity jets down the tube gaps.
It is further noted that, during steam generator cleaning (tube lane or inspection port access) high pressure and volume water is injected into the steam generator and is sprayed laterally relative to the longitudinal axis of the lance. That is, the water must be redirected 90 degrees to clean between tubes. Water turbulence from a 90 degree bend significantly increases the divergence of the exiting water jet.